The first and second cervical spine vertebrae (C1 and C2) are unique due to the presence of a synovial “pin joint” (referred to as the atlantoaxial joint), compared to the intervertebral discs in the lower cervical spine. As shown in FIG. 1A, the C2 vertebra 2 has a boney structure called the odontoid 4 which acts as the pin that sits within the ring of C1 vertebra 6. The odontoid 4 articulates against the anterior ring of C1 and is held in place by ligamentous structures. Atlantoaxial instability (AAI) occurs when there is excessive motion at the atlantoaxial joint most commonly caused by traumatic fracture of the odontoid. If left untreated, AAI can cause chronic pain, myelopathy and even death when even mild additional trauma is sustained to the destabilized joint segment. Odontoid fractures are classified into three types depending on the location of the fracture line in the C2 vertebra. Type II fractures, such as shown in FIG. 1B, are the most common and due to a higher risk of fracture non-union, surgical treatment is recommended. However, a surgical approach is not always possible, especially in the elderly patient population, due to concerns related to intraoperative blood loss, operating time, surgical invasiveness and recovery time. Given these concerns, only approximately 50% of the current elderly patient population is healthy enough to undergo surgery for AAI. Even when surgical treatment is possible, deciding the optimal treatment is not trivial and a common consensus has not yet been reached in clinical practice.
Posterior spinal fusion, in which an implant construct is used to hold adjacent vertebrae together until they heal into a single piece of solid bone, has become the most common surgical treatment for type II odontoid fractures in elderly patients when surgery is feasible. Since being introduced in 2001, the Harms construct has become the standard fixation for posterior fusion of the atlantoaxial segment due to good construct stability and high fusion rates.
FIGS. 2A and 2B show posterior and lateral views, respectively, of the use of the Harms construct in atlantoaxial fusion on a model of the upper cervical spine. The Harms construct consists of two polyaxial screws 14 that are inserted into the lateral masses 10 of C1, connected by titanium rods 16 to two additional polyaxial screws 18 that are inserted into the pedicles 12 of C2. Cap screws 20 are tightened down on the rods 16 to lock the construct in place and provide immediate stability to the joint until long term fusion occurs. The surgical procedure for implanting the Harms construct is very invasive, requiring a long incision and a dissection all the way down to the C1 lateral masses 10. The blood loss associated with this surgery tends to be very high due to disturbing the capillaries and venous plexus around the vertebral artery 22 and C2 nerve root 24. Additionally, placement of the C1 lateral mass screw 14 is risky due to the potential for injuring the vertebral artery 22 and often the C2 nerve root 24 is sacrificed to get a safer screw trajectory. Due to these surgical concerns, approximately half of the elderly patient population is not fit for surgery and must instead be treated conservatively with hard collar immobilization, ultimately leading to high fracture non-union rates and a permanent instability.
Attempts have been made both clinically and experimentally to develop new constructs and surgical techniques that better suit the needs of the C1/C2 segment. For example, Huang et al. (Posterior atlantoaxial fixation: a review of all techniques, The Spine Journal, Vol. 15, 2015, pp. 2271-2281) discusses, inter alia, various C1-C2 atlantoaxial stabilization/fixation techniques involving screws and clamps or hooks, such as C1-C2 apofix clamps, C1 hook combined with a C2 pedicle screw, and a C1 screw combined with C2 hooks. However, these techniques are not sufficiently stable (e.g., clamp slippage occurs frequently), result in pseudarthrosis, and/or are generally difficult to use in surgery. On the other hand, as shown in the posterior and lateral views of FIGS. 3A and 3B, respectively, and as reported in Dorward and Wright (Seven Years of Experience With C2 Translaminar Screw Fixation: Clinical Series and Review of the Literature, Neurosurgery, Vol. 68, No. 6, June 2011, pp. 1491-1499), C2 translaminar screws 26 have been used with excellent clinical success to replace C2 pedicle screws 18 in the Harms construct and thus eliminate the risk of a C2 screw injuring the vertebral artery 22. Although C2 translaminar screws 26 have reduced risk in posterior fusion procedures, the surgery remains equally invasive as the Harms procedure with regard to blood loss and operating time because of the continued use of C1 lateral mass screws 14.
It is therefore desirable to provide a spinal fusion fixation device that does not suffer from the above drawbacks.
Advantages of the present invention will become more fully apparent from the detailed description of the invention below.